In a wireless communication system, a base station communicates with a plurality of remote terminals, such as cellular mobile telephones. Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) are the traditional multiple access schemes for delivering simultaneous services to a number of terminals. The basic idea underlying the FDMA and TDMA systems is based upon sharing the available resource, respectively as several frequencies or as several time intervals, in such a way that several terminals can operate simultaneously without causing interference.
In contrast to these schemes using frequency division or time division, Code Division Multiple Access (CDMA) allows multiple users to share a common frequency and a common time channel by using coded modulation. More precisely, as is well known to the person skilled in the art, a scrambling code is associated with each base station, and this makes it possible to distinguish one base station from another. Furthermore, an orthogonal code, known by the person skilled in the art as an OVSF Code, is allotted to each remote terminal (such as, for example, a cellular mobile telephone). All the OVSF codes are mutually orthogonal, thus making it possible to distinguish one remote terminal from another.
Before sending a signal over the transmission channel to a remote terminal, the signal has been scrambled and spread by the base section using the scrambling code of the base station and the OVSF code of the remote terminal. In CDMA systems, it is again possible to distinguish between those which use a distinct frequency for transmission and reception (CDMA-FDD system), and those which use a common frequency for transmission and reception but distinct time domains for transmission and reception (CDMA-TDD system).
Third-generation terminals, such as cellular mobile telephones, must be compatible with the UMTS standard, that is, they must be capable of operating under various wireless transmission standards. Thus, they will have to be capable of operating in a system of the FDMA/TDMA type, for example according to the GSM or GPRS transmission standard, or else in communication systems of the CDMA-FDD or CDMA-TDD type, for example by using the UTRA-FDD, UTRA-TDD or IS-95 transmission standards.
The invention thus applies in particular to all terminals or components of wireless communication systems, such as cellular mobile telephones for example, regardless of the transmission standard used. This is whether the latter provides for constant envelope modulation (GSM and DCS systems, for example) or variable envelope modulation (systems of the CDMA type), although the invention is especially advantageous with respect to variable envelope modulation systems.
The radio frequency transmission circuitry of a component of a wireless communication system, such as a cellular mobile telephone for example, comprises a power amplifier for amplifying the signal to a sufficient level for transmission. In systems operating according to the CDMA standard, which exhibits variable envelope modulation, use is made of linear transmission circuitry that makes it possible to resend the amplitude of the signal without distortion.
One approach for embodying the power amplification of the transmission circuitry includes using amplification circuitry of the delta-sigma type. An example of such an architecture is described, for example, in U.S. Pat. No. 5,777,512. Amplification circuitry of the delta-sigma type intrinsically exhibits the advantage of being more competitive in terms of efficiency than conventional linear amplification means. However, the use of amplification circuitry of the delta-sigma type exhibits drawbacks that will now be discussed.
Such amplification circuitry comprises frequency selector networks that make it possible to adjust the position of the zeros of the noise transfer function, that is, to adjust the frequencies at which the quantization noise is in theory eliminated. Also, traditionally, these zeros are placed in the useful or desired transmission band in which the signal is situated, so as to comply with the signal/noise ratio required by the transmission standard used.
It is necessary to comply with noise templates defined by the transmission standards, and outside the useful band of the signal, the noise must not exceed a certain level of energy so as not to disturb other transmissions/receptions using different transmission standards. Also, this noise results, in particular, from the quantization noise produced by the delta sigma amplifier and from the noise being added to the input signal of the amplifier.
Since the major portion of the quantization noise is pushed out of the useful band of the signal, it is then necessary to make provisions at the output for the amplification circuitry of delta-sigma type. That is, one or more post-amplifier filters are outside the useful signal band, and their function in particular, includes eliminating the quantization noise.
Moreover, there are filtering constraints on the elements of the transmission circuitry that are upstream of the delta sigma amplifier so as to limit the noise being added to the input signal of the amplifier. Furthermore, the dynamic constraint at the level of the transmission antenna is significant. This then results in a constraint on the signal level at the output of the power amplification circuitry (by way of indication, the maximum level required at the output of the amplifier in the WCDMA standard is 27 dBm), and consequently on the signal level at the input of the amplifier.
At present, with a typical gain of the amplifier being 25 dB, the maximum level of the signal at the input of the amplifier has to be 2 dBm in the WCDMA standard. This has a significant impact on power consumption. Thus, not only does the input dynamic swing of the power amplification circuitry have to be high, but the output power of the mixer upstream of the power amplification device also has to be considerable. This also is a penalty in terms of power consumption.